Carissa,
You are correct in noticing that the amount of salt in solution can
affect the evaporation
rates of water. When salt is dissolved in water, each salt molecule breaks
up into one sodium ion
(Na+) and one Chloride ion (Cl-). Just like magnets, charged particles are
attracted to other
particles of opposite charge that might also be floating around in
solution.
What you may not be
aware of is that water molecules can also act like weak magnets and have
both positively and
negatively charged ends. When water molecules evaporate they have to absorb
enough energy
from their surroundings to break free of the attractive forces of the other
molecules around them.
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When salt is dissolved in the water however, much stronger magnetic forces
are present to hold
the water molecules together. To break free (or evaporate) the molecules
will require more
energy than before the salt was added.
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For your experiment, you are adding the same amount of energy to each
container but now the
molecules in the salt water need more energy per molecule to break free.
This means that fewer
molecules will have evaporated from the salted solution. Another
experiment
that demonstrates
this phenomenon is the temperature at which water boils when it is salted
as
opposed to when it
is not salted. Good Luck!!